Re: [PATCH] lib/stackdepot: always do filter_irq_stacks() in stack_depot_save()

[Vlastimil Babka <vbabka@suse.cz>]

On 11/30/21 10:57, Marco Elver wrote:
> The non-interrupt portion of interrupt stack traces before interrupt
> entry is usually arbitrary. Therefore, saving stack traces of interrupts
> (that include entries before interrupt entry) to stack depot leads to
> unbounded stackdepot growth.
> 
> As such, use of filter_irq_stacks() is a requirement to ensure
> stackdepot can efficiently deduplicate interrupt stacks.
> 
> Looking through all current users of stack_depot_save(), none (except
> KASAN) pass the stack trace through filter_irq_stacks() before passing
> it on to stack_depot_save().
> 
> Rather than adding filter_irq_stacks() to all current users of
> stack_depot_save(), it became clear that stack_depot_save() should
> simply do filter_irq_stacks().

Agree.

> Signed-off-by: Marco Elver <elver@google.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks.

> ---
>  lib/stackdepot.c  | 13 +++++++++++++
>  mm/kasan/common.c |  1 -
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index b437ae79aca1..519c7898c7f2 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -305,6 +305,9 @@ EXPORT_SYMBOL_GPL(stack_depot_fetch);
>   * (allocates using GFP flags of @alloc_flags). If @can_alloc is %false, avoids
>   * any allocations and will fail if no space is left to store the stack trace.
>   *
> + * If the stack trace in @entries is from an interrupt, only the portion up to
> + * interrupt entry is saved.
> + *
>   * Context: Any context, but setting @can_alloc to %false is required if
>   *          alloc_pages() cannot be used from the current context. Currently
>   *          this is the case from contexts where neither %GFP_ATOMIC nor
> @@ -323,6 +326,16 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  	unsigned long flags;
>  	u32 hash;
>  
> +	/*
> +	 * If this stack trace is from an interrupt, including anything before
> +	 * interrupt entry usually leads to unbounded stackdepot growth.
> +	 *
> +	 * Because use of filter_irq_stacks() is a requirement to ensure
> +	 * stackdepot can efficiently deduplicate interrupt stacks, always
> +	 * filter_irq_stacks() to simplify all callers' use of stackdepot.
> +	 */
> +	nr_entries = filter_irq_stacks(entries, nr_entries);
> +
>  	if (unlikely(nr_entries == 0) || stack_depot_disable)
>  		goto fast_exit;
>  
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 8428da2aaf17..efaa836e5132 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -36,7 +36,6 @@ depot_stack_handle_t kasan_save_stack(gfp_t flags, bool can_alloc)
>  	unsigned int nr_entries;
>  
>  	nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 0);
> -	nr_entries = filter_irq_stacks(entries, nr_entries);
>  	return __stack_depot_save(entries, nr_entries, flags, can_alloc);
>  }
>  
>